A method and a device for mapping Ethernet code blocks to an optical transport network (OTN) for transmission are provided. In the method, 10G Ethernet code blocks are mapped to an OTN frame corresponding to an optical channel payload unit 2 (OPU2) as a bit stream within a justification range according to an asynchronous bit-stream mapping manner, an identifier for indicating the asynchronous bit-stream mapping manner is set at an overhead (OH) portion of the OTN frame, an offset of a certain code block in the bit stream relative to a certain position of the OTN frame is recorded, and an indicator for indicating the offset position is set. Thus, specific solutions for mapping 10G Ethernet code blocks having an encoding rate lower than a minimum payload bandwidth of the OPU2 to the OTN for transmission is provided without changing the mature OTN architecture.